The present invention relates to multi-function AC induction motor control, in general, and more particularly to microcomputer-based gating of such a control system.
Solid-state AC motor starter circuits are generally known. See for instance "Design and Application of a Solid-State AC Motor Starter" by John Mungenast in IEEE Transactions, vol. IA-12, No. 1, 1976, pages 39-42. A most commonly used mode of starting an induction motor is by reduced-voltage. See for instance "Reduced-Voltage Starting of Squirrel-Cage Induction Motors" by Frank M. Bruce, R. J. Graefe, Arthur Lutz and M. D. Panlener, in IEEE Transactions, vol. IA-20, No. 1, 1982, pages 46-55.
In an AC motor drive, start-up requires a progressive increase of the voltage applied to the motor, through a controlled gating of the thyristors of the associated static converter. This technique has been shown in U.S. Pat. Nos. 4,384,243 and 4,482,852, both of Nicholas G. Muskovac. These two patents are hereby incorporated by reference. Once the motor is running, there is a need to adjust further the gating of the thyristors in order to match loading conditions. For that purpose, gating of the thyristors has been dynamically controlled in order to achieve energy saving as typically shown in the afore-stated abandoned Elms patent application.. To this effect the delay angle of firing of the thyristors is increased (which means that the voltage applied to the motor is decreased) if both the motor voltage and the motor current (as sensed) are increasing or decreasing and, conversely, the delay angle is decreased (and the voltage to the motor increased) if motor voltage and motor current vary in opposite directions.
In the multi-function control system described in the N. G. Muskovac et al. cross-referenced patent application, control of the thyristors for an AC motor drive is effected from a common control input for the various functions involved in such a system, typically starter, energy saver, protection. With the complex functions now installed for dynamic control of a motor drive, the gate pulse generator should be able to respond with good accuracy, to adapt to frequency variations and to track immediately any change in the applied delay angle. This has been attempted by using computer techniques. In this respect, it is most desirable to be able to accommodate such complexity of response with the availability of a standard and inexpensive microcomputer.
Computer-based digital control of thyristors is known. See for instance U.S. Pat. Nos. 4,577,269 of A. Abbondanti and 4,427,933 of Paul W. Wagener et al. These two patents show solutions to the problem of determining with a microcomputer at the last moment the instant of firing for the thyristors, thereby allowing a fast response to change and leaving time for other utilizations of the microcomputer.